This invention relates to selected coal tar compositions which are useful as lubricant basestocks in traction fluids. More particularly, the invention is directed to a traction fluid composition having a lubricant basestock which comprises hydrogenated coal tar containing a saturated fraction having a major portion made up of multiring components of at least three rings and an aromatic fraction which comprises at least about 12% by weight of said fluid and which fraction contains at least 50% by volume of multiring aromatic components.
Traction fluid is a term used to identify a class of lubricants that give superior performance in traction drives. A traction drive transfers force from one rotating shaft to another through a rolling contact. Efficient transfer requires that a minimum amount of slippage occurs. This property is measured by the traction coefficient which is defined as force transmitted divided by the normal force which keeps the rolling members in contact.
The coefficient of traction as defined above, has been one of the prime measurements used in defining useful traction fluids. Various studies have been made attempting to define the type of structures associated with higher traction properties. Thus, some of the more suitable structures which have been reported include U.S. Pat. No. 3,411,369 which discloses fused, saturated carbon containing rings; U.S. Pat. No. 3,440,894 which discloses organic compounds containing a saturated carbon containing ring or an acyclic structure having at least three quaternary carbon atoms; U.S. Pat. Nos. 3,595,796 and 3,598,740 disclose the use of selected naphthenes and branched paraffins; and U.S. Pat. No. 3,843,537 disclosed the use of naphthenes, partially saturated precursors of naphthenes, hydrorefined mineral oils, polyolefins and branched paraffins.
Generally, the structures defined in the literature as having good traction properties have not included aromatic constituents. This is exemplified by U.S. Pat. Nos. 3,595,796; 3,598,740 and 3,843,537 which indicate the general undesirability of aromatic unsaturation as it relates to traction properties and the need to limit aromatic content to very low levels.